Organic EL devices using organic substances are expected to be useful as the inexpensive full color display device of the solid light emission type having a great area, and various developments have been made. In general, an EL device is constituted with a light emitting layer and a pair of electrodes disposed at both sides of the light emitting layer. For the light emission, electrons are injected at the side of the cathode, and holes are injected at the side of the anode when an electric field is applied. The electrons are combined with the holes in the light emitting layer to form excited states, and the energy formed when the excited states returns to the ground state is discharged as light.
Conventional organic EL devices require greater driving voltages and exhibit smaller luminances of emitted light and smaller efficiencies of light emission than those of inorganic light emitting diodes. Moreover, marked deterioration in the properties takes place, and the devices have not been used in practical applications. The properties of the organic EL devices are being improved gradually, but a greater luminance of emitted light and a greater efficiency of light emission under application of a low voltage are required.
To overcome the above problem, for example, a device using a compound having the benzimidazole structure as the light emitting material is disclosed in Patent Reference 1, and it is described that the device emits light at a luminance of 200 nit under a voltage of 9 V. In Patent Reference 2, a compound having the benzimidazole ring and the anthracene skeleton structure is described. However, an organic EL device exhibiting a greater luminance of emitted light and a greater efficiency of light emission than those obtained by the organic EL devices using the above compounds is required.
[Patent Reference 1] U.S. Pat. No. 5,645,948
[Patent Reference 1] Japanese Patent Application Laid-Open No. 2002-38141